1. Field of the Invention
This invention relates to a reversible electrochemical electrode, applicable for electrochemical elements such as batteries, sensors, or display devices.
2. Description of the Prior Art
In recent years, net rechargeable batteries have been investigated, which have a negative electrode comprised of alkali metal, alkaline earth metal or a complex thereof and a positive electrode comprised of a conductive polymer.
When the battery, having the positive and negative electrodes arranged face to face in an electrolyte, is charged, cations of the electrolyte are deposited on the negative electrode and anions of the electrolyte are injected into the positive electrode (the positive electrode is doped). When the battery is discharged, the opposite reaction occurs. Thus, charging and discharging can be repeated reversibly. The positive electrode material which is capable of doping and dedoping with anions is made by electrolytic polymerization of pyrrole, thiophene, aniline or such heterocyclic compound, or a monomer of an aromatic compound.
As the negative electrode material, lithium is most commonly used, since the electromotive force of the unit cell thereof exceeds 3 V, and is practically favorable. However, lithium readily reacts with oxygen and humidity in the air and loses its activity, and, to prevent the contact of the electrode with the outer atmosphere, great caution has been exercised. For example, a material of polyacene or such pyropolymer, or graphite fluoride intercalated with lithium ions has been proposed.
The doping of the conductive solid polymer for the electrode material with ions is achieved by a chemical or electrochemical method. Whether the doping of the conductive polymer with cations or anions goes well depends on the interaction of the conductive polymer and dopant. In addition, the strength of the interaction is decided by the electrochemical potential or electron affinity.
With these methods, however, the dopant density has certain limits. For example according to Denryoku-Chuo-Kenkyusho Hokoku (Report of Central Laboratory on Electric Power of Japan), No. T89046, the doping of polyacene was only 4% for p-type material and 5.5% for n-type material at the most.
Also, most of the conductive polymer cannot be doped to n-type.
Even the conductive material capable of being doped to p-type to some extent, such as pyrrole or aniline for example, does not attain satisfactory performance as the cell.